The present invention relates to metallurgical furnaces in general and, more particularly, to improvements in the design of workpiece-engaging elements used in walking beam and so-called pusher-type furnaces.
Metallurgical furnaces are employed for continuous transport of ingots, slabs, rods, bars or like metallic workpieces through a heating chamber. The so-called pusher-type furnaces employ elongated rails along which the workpieces are pushed through the heating chamber whereby the workpieces are heated from above and from below. A disadvantage of such a furnace is that the surfaces of the workpieces are likely to be scratched or otherwise damaged through sliding movement along the rails which is particularly undesirable when the furnace is employed for the treatment of expensive and highly-sensitive metallic workpieces.
It is known that in such furnaces contact between the workpieces and the rails along which they are pushed results in the formation of strip-like undercooled zones in the region of the workpiece surfaces which contact the support. It will be appreciated that the rails must be cooled, usually by circulating cooling fluid through interior passages of the rails, so that they will not themselves be deformed at the elevated temperatures of the furnace. These undercooled zones or dark strips are very undesirable because they adversely affect material characteristics, particularly during subsequent rolling.
In order to eliminate this problem, the prior art has proposed so-called "riders" or workpiece-engaging elements composed of heat-insulating material and which are fixedly mounted on the rails and extend over the entire length thereof. The workpieces are supported on these riders for the purpose of preventing undercooling of portions of the workpieces which would otherwise be in direct contact with the internally-cooled rails.
However, the prior-art proposals have not proven altogether satisfactory. The higher the requirement which is imposed for preventing the development of undercooled zones in the workpiece, the more complex must be the construction of the riders and the more necessary it is to use expensive highly heat-resistant materials. The construction of riders currently in use are characterized in that they are generally massive and have rather large heights, considerable widths and are constituted of more than one part. For example, it is frequently necessary under circumstances where the temperatures are extremely elevated to use ceramic material for at least the upper portions of the riders. One-piece riders are generally only useful in furnaces where the anticipated temperature ranges are not very high.
In one known application, the rider comprises an upwardly-projecting central portion of rather large height which is flanked by side flanges so as to roughly resemble an inverted U-shaped configuration. This rider sits on the rail by embracing opposite sides thereof. However, the upper workpiece-engaging contact surface of this rider is so greatly spaced from the cooling rail that it is heated to very elevated temperatures which, in turn, means that the upper structural limit of the material of the upper workpiece-engaging portion is soon reached.
In addition, the workpiece-engaging upper surface of the U-shaped rider is at least as wide as the upper width of the elongated rail. This is very undesirable because uniform heating of the underside of the workpiece is possible only if the access of heat to the workpiece is blocked as little as possible between the riders and the rails. This requires that the riders and rails be as small as possible and be well insulated. Since the U-shaped rider embraces opposite sides of the rail, it is apparent that the dimensions of the rider and rail are undesirably large, thereby screening heat from the underside of the workpiece.
In another known application, the rider has a circular configuration so that only line contact is made between the rider and the workpiece. However, this is not satisfactory since the upper structural limit at which the rider will become damaged by heat is not reduced. Also, the rail is partially surrounded by ceramic insulating sleeves which leave a wide gap therebetween to receive the rider. This is disadvantageous because contaminants enter the gap and, over time, tend to expand and destroy the ceramic-insulating sleeves. Still another drawback is that side extensions are used to engage opposite sides of the rider so as to partially overlap the gap. However, the entry of contaminants is still not altogether prevented.